Just in case the above won;t work without at least one plant in "Your Plant List", try ading you first plant.
1. Find any entry in the Plant Database that you're interested in.
At the top of the far-left, blue menu bar, click "Plant Database".
Type some names, common or scientific, into the "Search for a plant" box.
Enter
Pick one that's interesting,
2. Click on one of the search results, which will take you to that plant's database entry.
3. Within the entry for one plant variety in the Database, page down, down, down. All he way down, past photos and comments, Event Reports and Related threads. You're getting close when you see "Adding Actions" and "Editing Actions:".
Below them, find:
Plant List
Dark blue button: "Add this plant to your list".
Click the dark blue button: "Add this plant to your list".
Now you have a Plant List with one entry, and you can add categories.
Maybe some of those steps look different the very first time you click on them. If so, let me know where these directions fail and I'll try again.